In computing systems, such as desktop computers, portable computers, personal digital assistants (PDAs), servers, and others, storage devices are used to store data and program instructions. One type of storage device is a disk-based device, such as a magnetic disk drive (e.g., a floppy disk drive or hard disk drive) and an optical disk drive (e.g., a CD or DVD drive). Disk-based storage devices have a rotating storage medium with a relatively large storage capacity. However, disk-based storage devices offer relatively slow read-write speeds when compared to operating speeds of other components of a computing system, such as microprocessors and other semiconductor devices.
Another type of storage device is a solid state memory device, such as a dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, and electrically erasable and programmable read-only memory (EEPROM). Although solid state memory devices offer relatively high read-write speeds, usually on the order of nanoseconds, they have relatively limited storage capacities.
With improvements in nanotechnology (technology involving microscopic moving parts), other types of storage devices are being developed. One such storage device (referred to as “probe-based storage device”) is based on atomic force microscopy (AFM), in which one or more microscopic scanning probes are used to read and write to a storage medium. Typically, a scanning probe has a tip that is contacted to a surface of the storage medium. Storage of data in the storage medium is based on perturbations created by the tip of the probe in the surface of the storage medium. In one implementation, a perturbation is a dent in the storage medium surface, with a dent representing a logical “1,” and the lack of a dent representing a logical “0.”
Effectively, the probe produces a signal that is related to the depth of a dent (lack of a dent is associated with zero depth). For optimal detection of a dent, it is desired that the probe tip be fully engaged in the dent. However, the track followed by a probe tip during a read operation may not follow a track that is aligned with centers of dents formed during write operations. In other words, the track followed by the probe tip during a read is offset from the track followed by the probe tip during a write. The track offset can be caused by several factors, such as shock or vibration during use of the storage device, manufacturing tolerances, and so forth. The track offset results in misalignment between the probe tip and a dent during a read operation. Misalignmnet causes the probe tip to pass close to the edge of a dent, rather than through the center of the dent. If the probe tip passes closer to the edge of a dent than through the center of the dent, then the probe tip may not drop as much into the dent. The shallower depth detected by the probe tip causes the amplitude of the signal produced by the probe to be smaller than if the probe tip is fully engaged within the dent. The smaller signal amplitude is more difficult to detect accurately, particularly in the presence of noise that typically exists in a storage device during operation. To remedy the situation, very tight tolerances may have to be placed on tracks to be followed by probe tips during write and read operations in the storage medium. Such tight tolerances may be difficult to achieve and may result in lower yields and higher manufacturing costs.